Refrigeration systems have been known in the past in which a refrigerant is circulated in a refrigerant circuit to perform a refrigeration cycle. Such a refrigeration system has been widely used in various applications such as air conditioners. For example, JP-A-2000-241033 (hereinafter referred to as “Patent Document I”) discloses a refrigeration system including a compressor for the compression of refrigerant and a power recovery expander for the expansion of refrigerant. More specifically, in a refrigeration system shown in FIG. 1 of Patent Document I, the expander is coupled through a single shaft to the compressor and power obtained in the expander is used to drive the compressor. In addition, in a refrigeration system shown in FIG. 6 of Patent Document I, an electric motor is coupled to the compressor and an electric power generator is coupled to the expander. In this refrigeration system, the compressor is driven by the electric power generator to compress refrigerant. On the other hand, the electric power generator is driven by the expander to generate electric power.
For example, JP-A-2005-299632 (hereinafter referred to as “Patent Document II”) discloses a fluid machine including an expander and a compressor which are coupled together through a single shaft. In the fluid machine disclosed in Patent Document II, a compression mechanism as a compressor, an expansion mechanism as an expander, and a shaft for connection between the compression mechanism and the expansion mechanism are all accommodated in a single casing. In addition, in this fluid machine, there is formed in the inside of the shaft an oil supply passageway, and lubricant oil accumulated in the bottom of the casing is supplied through the oil supply passageway to the compression mechanism and to the expansion mechanism.
Moreover, JP-A-2005-002832 (hereinafter referred to as “Patent Document III”) discloses a so-called “hermetic compressor”. In this hermetic compressor, a compression mechanism and an electric motor are accommodated in a single casing. In addition, in this hermetic compressor, an oil supply passageway is formed in a drive shaft for the compression mechanism, and lubricant oil accumulated in the bottom of the casing is supplied through the oil supply passageway to the compression mechanism. The refrigeration system shown in FIG. 6 of Patent Document I may employ such a type of hermetic compressor.